Many power applications require precise switching and linear conduction of currents between a source and a load to ensure adequate performance. Such applications include drive circuitry for electrical motors or lamps and medical imaging systems, such as a magnetic resonance imaging (MRI) system. In MRI systems, MRI data can be detrimentally affected if a scan is not precisely controlled. In general, in an MRI system, an MRI scan is performed in accordance with an imaging protocol which includes one or more predefined pulse sequences. A pulse sequence defines the manner in which gradient magnetic fields are generated in the scanning device, which, in turn, govern parameters of the scan, such as slice orientation, frequency encoding and phase encoding. Failure to precisely control the generation of the gradient fields results in poor imaging data that may exhibit, for example, geometric distortion or poor spatial resolution.
In an MRI scanner, a primary magnetic field, B0, typically is produced by a superconducting electromagnet. Gradient magnetic fields are produced in B0 by a gradient coil assembly which typically includes three pairs of coils. Typically, the first pair of coils is configured to generate a gradient in the magnetic field along the physical x-axis of the scanner. Similarly, the second and third pairs of coils are configured to generate gradients in the magnetic field along the physical y-axis and z-axis of the scanner, respectively.
MRI scanners may offer improved imaging performance and image resolution by incorporating twin gradient coil sets that include a “whole body” coil set for generating “coarse” MRI data during a scan and a smaller, “supplemental” coil set for generating “fine” MRI data. Each of the twin gradient coils sets includes an x-axis coil pair, a y-axis coil pair, and a z-axis coil pair. Both coil sets contribute to the MRI data measurements; however, only one coil set is energized at a time. Thus, to improve imaging performance and image resolution, the whole body coil set is energized to perform a first scan in accordance with a predefined pulse sequence, and then the supplemental coil set is energized to perform a second scan. The coil sets may be alternately energized for further scans as may be required for the particular imaging application.
Switching between the coil sets may be performed in a variety of manners. For example, a switch may be coupled, between a source that provides the power to energize the coils. The switch may be manually manipulated (e.g., mechanically or electrically), which can be inconvenient and slow, or may be automatically manipulated (e.g., via a software control program). Further, the switch may be a mechanical switch (e.g., a contactor) or an electronic switch (e.g., a transistor, diode, etc.). However, regardless of the method of manipulation or the type of switch used, well-controlled, high performance MR imaging requires that the gradient coils be driven in accordance with the predefined pulse sequence in a continuous, strong and reproducible manner. Thus, for example, the switch must be capable of conducting high amplitude gradient currents that transition between positive and negative values, as well as between a positive or negative value and a zero (or very low amplitude) value. That is, to ensure MR imaging performance, the switch ideally should exhibit a linear, or uninterrupted, conductive state throughout all portions of the pulse sequence during which the gradient current flows.
One type of linear switch is a mechanical contactor that can be switched between conductive and non-conductive states by, for example, either a manual or automatic remote command. Because the contactor conducts current linearly in both directions, no special measures or circuitry is required to ensure proper steering of the load current between the gradient coil assembly and the gradient coil drive circuitry. Contactors, however, are large, acoustically noisy, and sensitive to the magnetic field generated by the scanner. Thus, the use of contactors can present complexities in the physical mounting, shielding and pack aging of the contactors and other associated components. Further, the switching speed of a contactor between conductive and non-conductive states is slow, e.g., typically 5-15 milliseconds. Moreover, switching a contactor often causes arcing which can reduce the contactor's useful life though arcing can be controlled through appropriate protection circuitry or via mechanical structures, such control introduces additional complexities, which can translate to additional costs and decreased reliability. As a result a mechanical switch (e.g., a contactor) may not be the optimal choice.
Other alternatives include electronic switches, such as a transistor, a diode, a thyristor, etc. Electronic switches, however, may not be characterized by a linear conductive state. That is, the conductive characteristics of an electronic switch may be dependent on the magnitude of th e current flow through the switch. Thus, to ensure linearity in the conduction of current between the load (e.g. gradient coils) and the drive circuitry, additional circuitry may be required to maintain linear current flow during periods in which the current transitions between positive and negative values, and/or as the current transitions from a positive or negative value to a substantially no-current flow condition.
An exemplary topology of an electronic switch includes a transistor coupled with a diode bridge. In this topology, the diodes in the bridge steer the current between positive and negative values and/or to a near-zero current flow condition. However, when such a topology is used in an application that requires high levels of current flow, both the transistor an d the diode bridge must be power components, thus consuming a significant amount of packaging space. Further, because the current at any time must flow through a transistor junction and/or two diode junctions, a significant amount of energy is dissipated in the components, requiring a complex heat sink scheme, such as a water-cooled mounting plate, fans, fins, etc. Thus, such a transistor/diode bridge topology may not be the optimal choice for power applications.
Accordingly, there is a need for a switching assembly to selectively couple a source (e.g., a gradient amplifier) to a load (e.g., a gradient coil assembly). Such a switching assembly would include a switching device having a conductive state in which a current is conducted in an uninterrupted, or continuous, manner between the source and the load. Further, such a switching assembly should dissipate a minimum amount of heat, have a minimum number of components, switch quickly, quietly and reliably, be relatively immune to the effects of a magnetic field, and generate a limited amount of electromagnetic interference.